Cambridge, Massachusetts, is home to Homer and Mary Kelly, Harvard University, the Mount Auburn Cemetery, and Leonard Sheldrake. Leonard, Homer's friend, often compares the many-faceted Cambridge to a favorite engraving, filled with strange power and wonder, by the twentieth-century Dutch artist Maurits Escher. So he is thrilled when Cambridge hosts an Escher exhibit. There, Leonard is smitten by a mysterious woman in a green coat named Frieda who...